The present invention relates to controllers for LED devices and more particularly to dimming controllers for displays backlit by LED displays.
A light emitting diode, or LED, comprises a diode that emits visible light when current passes through it. LEDs have several applications. Certain display devices, for example, but not limited to, aircraft cockpit displays, use an array of LEDs to backlight and illuminate a liquid crystal display (LCD). Controlling the amount of light emitted by the LED array is desirable to adjust the brightness of the display. The brightness level impacts the ease with which the display may be viewed under certain lighting conditions, such as bright sunlight; as well as individual viewer comfort level with the display.
In some applications, the brightness level is more than a convenience factor. For example, in the aviation environment, if the display is illuminated too brightly at night, the excessive brightness may adversely impact the pilot""s night vision. Impaired night vision impacts the safety of flight.
The brightness level additionally affects the amount of power required to operate the device as well as the heat given off by the display. Power consumption affects the length of time the device can operate on battery power and the electrical load placed on the vehicle power supply systems. The heat given off by the display also affects what, if any, cooling of the display and surrounding equipment is required. Cooling devices add cost and complexity to equipment and systems. In aircraft/spacecraft applications, cooling systems add unwanted additional weight to the vehicle. Furthermore, if the display generates too much heat, touching or otherwise operating the display may cause discomfort, and in extreme cases, present a fire hazard, to the user.
The amount of light emitted by the diode can be controlled by controlling the amount of power supplied to the diode where power equals voltage times current (P=V*I). In certain prior art devices, a microprocessor device is coupled to drive circuitry that controls the LED display brightness. In such designs, a technique known as pulse width modulation (PWM) is used to control the power supplied to the device. Under control of the microprocessor, the drive circuitry supplies current to the LED for a predetermined amount of time, or one pulse width. In this manner, by varying the number of pulses received and the width of the pulses, the total power supplied to the LED, and hence the brightness can be controlled.
Thus, in theory, the lowest luminance level which can be achieved by the display is limited only by the resolution with which the pulse frequency and width can be conveyed from the modulator to the LED circuit. In practice, however, these low brightness levels can be difficult to achieve. The LED devices which comprise the display experience performance changes as a function of temperature. In addition, the LED devices may not have uniform electrical properties. These nonuniformities result in different power levels required to operate individual ones of the LED devices. Precise control of the array brightness in prior art designs is therefore difficult especially at low brightness levels. Furthermore, the human eye is especially adept at perceiving light emitted from the diode even at low power levels. This fact further exacerbates the nonlinearities in luminescence present in prior art devices. Thus, it is not presently possible to control the brightness of the LED display with the precision desired.
The present invention recognizes the problems of the prior art and provides a solution thereto. In particular, the present invention provides a method and apparatus for illuminating a display using an array of LEDs that provides for improved safety, efficiency and performance over prior art designs. The present invention enables precise control of the brightness level of the LED display even at low luminance levels.
According to one aspect of the present invention, the present invention includes a current limiter that bounds the amount of current that may be supplied to the entire array. Fire hazards and poor operation due to current runaway conditions are thereby prevented. In addition the current limiter of the present invention minimizes the size power supply required to operate the array and the cost of associated components. Operation of the LED array is also made more robust since power supply fluctuations and manufacturing tolerance variations in circuit components do not impact operation of the display.
According to another aspect of the present invention, the invention includes a controller, or control circuit, that controls display brightness. In a preferred embodiment of the invention, the control circuit includes a control signal buffer and an array driver that operate to control LED current in a manner linearly proportional to the level commanded by the control pulse. When pulse width modulation is used to control display brightness, the control circuit of the present invention operates to respond by switching the current drawn through the LED array within the time frame of the shortest duration pulse. Precise control of display brightness is achieved at even the lowest of commanded brightness levels.
Further features and advantages of the present invention will be described in detail below with reference to the drawings.